Evaluation of constructed wetland systems toward naturalization of the elemental composition and microbial community in wastewater treatment plant effluent

Numerous studies have been conducted on the advanced treatment of wastewater treatment plant (WWTP) effluent by constructed wetlands (CWs); however, no studies have focused on the effect of the naturalization function of CWs on WWTP tailwater, i.e., reducing imbalances in the elemental composition and microbial community, compared with natural surface water (NSW). Therefore, the possible naturalization functions of CWs were evaluated by comparing the elemental composition, microbial structure and function in NSW with the influent and effluent of CW. Surveys were conducted in seventeen full-scale wetlands constructed to treat WWTP tailwater in eastern China, and the largest lake in the region, Taihu Lake, was selected as the reference for NSW. The imbalance index (IMI) was proposed to characterize the difference in the elemental proportions between tailwater and the references. The results showed that the IMI values for the WWTP effluent ranged from 0.51 to 0.10 and from 0.95 to 0.37 for macroelements and microelements, with average values of 0.30 and 0.62, respectively. When macronutrient IMI>0.27 or micronutrient IMI>0.43, the CWs alleviated the element imbalance; otherwise, the CWs increased the IMI and aggravated the imbalance. With the Hoagland formula as the reference, a similar result was obtained. CW treatment reduced the difference in bacterial composition between tailwater and NSW. According to FAPROTAX and BugBase function prediction, CW treatment reduced the difference in the relative abundance of microbial functions related to C, N, and S cycles, pathogenicity and antibiotic resistance genes between tailwater and NSW. The CW area and hydraulic retention time were significantly positively correlated with the naturalization index. Therefore, the CW system can naturalize tailwater in terms of both elemental composition and microbial composition and function, indicating the necessity of re-evaluating CW functions in treating WWTP tailwater.